San Francisco

There’s geology underneath those streets

This page is under construction, should be finished by late February 2025.

Overview

All of the City and County of San Francisco are part of the North American plate, and are east of the San Andreas Fault. The San Andreas fault splits the peninsula along I-280, going offshore at Mussel Rock between Daly City and Pacifica. Maps easily reveal the San Andreas as the straight path between the San Andreas Lake /Crystal Springs Reservoir and Tomales Bay. Point Reyes coastal San Mateo coast is made up of rocks that have traveled hundreds of miles as the San Andreas Fault slides.

Before the transverse/sliding motion of the San Andreas Fault, the North American Plate was riding up over the subducting Farallon Plate (see animation below). The rocks underlying SF have been ‘scraped up’ off the Farallon Plate as the North American Plate rode up over it. These rocks have piled up in groups called terranes. Each terrane represents a different environment the rocks formed in: The easternmost SF terrane is the Alcatraz terrane (named such because similar rocks are found on Alcatraz Island), the Marin Headlands terrane (also found in the Marin Headlands), and the San Bruno Mountain terrane.

More on these terranes and melanges can be found below.

The animation on the right shows how, at about 15 Ma¹, the North American plate stopped riding up over the Farallon plate, and San Francisco changed from being a subduction zone to a transverse fault.

As terranes are accreted on the continental plate, every so often a break occurs that causes the trailing terrane to start subducting under the accreted terrane. We then get a series of angled terranes. As one terrane starts subducting under the previous accreted terrane, friction between the two starts grinding up the rocks along this contact. The resulting mixture is called a melange. These major terranes are separated by melanges, areas where the adjoining terrane rocks were ground and mixed up as one terrane was forced over the other. These melanges are less resilient that the terranes, and thus are often locations for landslides.

More details “Overview of Plate Tectonics and Tectonic Accretion” can be found on Page 31 of this document.

Historic imagery

The diagram on the right shows an approximation of a cross-section of the collision between the Farallon Plate and the North American Plate, showing the creation of the Coast Ranges as ocean sediments are accreted onto the North American Plate. Portions of each terrane are subducted to various degrees and the heat and pressure cements the rocks and generates the chemical reactions that defines metamorphosis.

Hikes

A collection of hikes I lead, most of these are on the AllTrails platform, that allows you to download maps using their free app on iPhone or Android. Basic map downloads are free; if you want more options there is a annual paid membership.

• Sutro Baths/Lands end/China Beach
• Rocks & Paths: Mission St BART to Glenn Park BART (this one is on RideWithGPS)

Major rock types found around San Francisco

Pillow basalt

As magma rises under the ocean (e.g. at mid-ocean ridges), it cools as it comes in contact with the water. If the extrusion of magma is slow enough, no explosions occur, and the magma cools into blob called ‘pillows.’ As one pillow starts to harden, the magma extruding near it will start to create another pillow (see image). This continues and can create vast regions of layer upon layer pillows.

Radiolarian Chert

As the spreading oceanic plate moves the pillows away from the center, microscopic organisms called radiolarians slowly drop out of the water column as they die. Radiolarian skeletons are made of silica, unlike most shells that are made of calcium. Over millions of years, as the oceanic plate continues to spread, massive layers of radiolarian skeletons continue to pile up.

Sandstone, Graywacke, and Turbidite Sandstone

Finally, as the conveyor belt of the moving oceanic crust approaches a shoreline, sediments from the shore start to settle on top of the chert. Smaller sediments float in the ocean waters longer than larger ones, so we find siltstone as the first/lowest layer. On top of that we’ll find sandstones. Even closer to shore we can find larger rocks and cobbles that made it off shore. In some areas, undersea landslides occur that create turbodite currents that mix up the size and result in turbodite sandstones.

Graywacke Sandstone FAQ, from National Parks Service.

Terranes ^{2 or 3}

We’ll examine the terranes from east to west, which is also the oldest to newest.

Name	Age (Ma1)	Rock types
Alcatraz terrane	95	graywacke shale
Hunters Point melange		ground up rocks from neighboring terranes
Marin Headlands terrane	100-200	pillow basalt radiolarian chert graywacke
City College melange		ground up rocks from neighboring terranes
Bolinas Ridge		graywacke sandstone
San Bruno Mountain terrane		graywacke shale

Alcatraz Terrane

Alcatraz Terrane can be found in San Francisco at Telegraph Hill.

Marin Headlands Terrane

Marin Headlands Terrane can be found in San Francisco at Twin Peaks.

An incredible slickenside can be found at Corona Heights Park. The summit at Corona Heights Park is chert.

Bolinas Ridge Terrane

A small portion of this terrane is visible at Lands End.

San Bruno Mountain Terrane

San Bruno Mountain terrane can be found at, no surprise here, San Bruno Mountain and west of City College.

Melange Zones

Hunters Point Melange Zone

Running between Marin Headlands and Alcatraz terranes.

City College Melange Zone

Running between Marin Headlands and San Bruno/Bolinas Ridge terranes, a landslide near Lands End is good evidence of the low stability of this region.

Resources

Websites

• San Francisco: Where the Plates Meet, Blogs of the European Geosciences Union
• Geology of San Francisco, 2018, Association Engineering Geologist (PDF format)

Books

• My favorite book is Geology of the San Francisco Bay Region, by Doris Sloan, 2006. It covers the Bay Area extensively, but is target at the novice looking to learn about geology. You can find it used, in your local public library, or new.
• A great list of trips that can be easily worked into hikes can be found in Geologic Trips; San Francisco and the Bay Area, by Ted Koningsmark, 1998. You can find it used or in your local public library (including ebook version) (I couldn’t find a source for new copies).

Videos

Footnotes